Active regions are treated in this review as a ``paradigm'' of a complex dynamical system . Active regions are formed by magnetic fibers escaping from the turbulent convection zone. Random movements of the feet of the fibers in the photosphere and emergence of new magnetic flux from the convection zone are responsible for the formation of neutral sheets and magnetic discontinuities inside the active region, which are the sites for magnetic dissipation. A simple model, based on the scenario of self organised criticality, reproduces many of the known flare characteristics. The same model is used to provide a large number of nanoflares, which can heat the corona. We also show that this complex, inhomogeneous active region, is an efficient accelerator and reproduce the observed dm spikes and type III bursts.